Sound Transmission Class (or STC) is an integer rating of how well a buildingpartition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI)ISO index or its related indices are used. As of 2012, these are defined in the ISO - 140 series of standards (under revision).

The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide.

The ASTM sound transmission loss test methods have changed every few years. Thus, STC results posted before 1999 may not produce the same results today, and the differences become wider as one goes further back in time–the differences in the applicable test methods between the 1970s and today being quite significant.[citation needed]

The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These Transmission Loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds, but much less so for amplified music, mechanical equipment noise, transportation noise, or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce. Alternatively, Outdoor-Indoor Transmission Class (OITC) is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.[citation needed]

The following sound isolation results and methodologies are presented with data that is measured within the standard frequency range specified by appropriate ASTM standards. Although it is worthwhile to discuss the utility of sound transmission loss data that lies outside the standard frequency range (especially in the low-frequency region), for simplicity results will be primarily be presented and discussed within these standard limitations.

Typical interior walls in homes (1 sheet of 1/2″ (13 mm) gypsum wallboard (drywall) on either side of a 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center with fiberglass insulation filling each stud cavity) have an STC of about 33.[1] When asked to rate their acoustical performance, people often describe these walls as "paper thin." They offer little in the way of privacy. Multi-family demising partition walls are typically constructed with varying gypsum wallboard panel layers attached to both sides of double 2x4 (90 mm) wood studs spaced 16" (406 mm) on-center and separated by a 1" (25 mm) airspace. These double-stud walls vary in sound isolation performance from the mid STC-40s into the high STC-60s depending on the presence of insulation and the gypsum wallboard type and quantity.[1] Commercial buildings are typically constructed using steel studs of varying widths, gauges, and on-center spacings. Each of these framing characteristics have an effect on the sound isolation of the partition to varying degrees.[2]

Adding absorptive materials to the interior surfaces of rooms (e.g. fabric-faced fiberglass panels, thick curtains) will result in a decrease of reverberated sound energy within the room. However, absorptive interior surface treatments do not significantly improve the sound isolation from one room to another through demising partitions over the typical frequency range measured currently.[3] Installing absorptive insulation (e.g., fiberglass batts, blow-in cellulose, mineral fiber batts) into the wall or ceiling cavities effects the sound isolation of the partition to varying degrees, depending on the framing configuration and joist or stud depth.[1] For example, the presence of type of insulation in single 2x4 wood stud framing spaced 16" (406 mm) on-center results in only a few STC points. In contrast, adding standard fiberglass insulation to an otherwise empty cavity in light-gauge (25-gauge or lighter) steel stud partitions can result in a nearly 10 STC-point improvement. As the stud gauge becomes heavier, the presence and type of insulation matters less.

The effect of adding multiple layers of gypsum wallboard to a frame also varies depending on the framing type and configuration.[4] Doubling the mass of a partition does not double the STC, as the STC is calculated from a non-linear decibel sound transmission loss measurement.[5] So, whereas installing an additional layer of gypsum wallboard to a light-gauge (25-ga. or lighter) steel stud partition will result in about a 5 STC-point increase, doing the same on single wood or single heavy-gauge steel will result in only 2 to 3 additional STC points.[4] Adding a second additional layer (to the already 3-layer system) does not result in as drastic an STC change as the first additional layer.[1] The effect of additional gypsum wallboard layers on double- and staggered-stud partitions is similar to that of light-gauge steel partitions. Due to increased mass, poured concrete and concrete blocks typically achieve higher STC values (in the mid STC 40s to the mid STC 50s) than equally thick framed walls.[6] However the additional weight, added complexity of construction, and poor thermal insulation tend to limit masonry wall partitions as a viable sound isolation solution in many building construction projects. Temperate climates and hurricane- or tornado-prone areas may, however, require the use of masonry walls for structural stability.

Structurally decoupling the gypsum wallboard panels from the partition framing can result in a large increase in sound isolation when installed correctly. Examples of structural decoupling in building construction include resilient channels, sound isolation clips and hat channels, and staggered- or double-stud framing. The STC results of decoupling in wall and ceiling assemblies varies significantly depending on the framing type, air cavity volume, and decoupling material type.[7] Great care must be taken in each type of decoupled partition construction, as any fastener that becomes mechanically (rigidly) coupled to the framing can short-circuit the decoupling and result in drastically lower sound isolation results.[8]

Sound damping tapes and other materials have been used to reduce both vibration and sound transmission through materials since the early 1930s.[9] Although the applications of sound damping was largely limited to defense and industrial applications such as naval vessels and aircraft in the past, recent research has proven the effectiveness of damping in interior sound isolation in buildings.[10] Constrained-layer damping gypsum wallboard panels increase sound isolation in building partitions by drastically reducing the vibration of panels and, incidentally, the radiation of sound through panels. The shear loading of a highly visco-elastic interlayer sandwiched between two more rigid constraining layers causes decreased displacement due to vibration, reducing the amount of sound energy radiated through a panel between enclosures.[11] Damped gypsum wallboard panels are effective in reducing sound transmission over a broad range of frequencies and especially useful for achieving high levels of speech privacy between partitions.[12]

Sound isolation metrics, such as the STC, are measured in specially-isolated and designed laboratory test chambers. It is important to note that there are nearly infinite field conditions that will affect sound isolation in situ when designing or remodeling building partitions and enclosures. Partitions that are inadequately or inappropriately sealed—that contain back-to-back electrical boxes, untreated recessed lighting, and unsealed pipes to name just a few—provide flanking paths for sound. Sound flanking paths include any sound transmission path other than the wall or ceiling partition itself. Great care and caution must be applied to any acoustically-treated building partition to ensure that the field sound isolation performance more closely approaches laboratory-tested values (see data from the National Research Council of Canada.[13])

Section 1207 of International Building Code 2006 states that separation between dwelling units and between dwelling units and public and service areas must achieve STC 50 (STC 45 if field tested) for both airborne and structure-borne. However, not all jurisdictions use the IBC 2006 for their building or municipal code. In jurisdictions where IBC 2006 is used, this requirement may not apply to all dwelling units. For example, a building conversion may not need to meet this rating for all walls.[citation needed]

In serious cases (e.g., a bedroom adjacent to a home theater room, and an inconsiderate nocturnal neighbor, to boot) a partition to reduce sounds from high-powered home theater or stereo should ideally be STC 70 or greater, and show good attenuation at low frequencies. An STC 70 wall can require detailed design and construction and can be easily compromised by 'flanking noise', sound traveling around the partition through the contiguous frame of the structure, thus reducing the STC significantly. STC 65 to 70 walls are often designed into luxury multifamily units, dedicated home theaters, and high end hotels.[citation needed]

STC

What can be heard

25

Normal speech can be understood quite easily and distinctly through wall

30

Loud speech can be understood fairly well, normal speech heard but not understood

35

Loud speech audible but not intelligible

40

Onset of "privacy"

42

Loud speech audible as a murmur

45

Loud speech not audible; 90% of statistical population not annoyed

50

Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed.